An electric conductor is generally made of aluminum, an aluminum alloy, copper or a copper alloy. However, aluminum has a low melting point of 660.degree. C. and exhibits no strength under a high temperature. An aluminum alloy also has similar problems. On the other hand, copper has a melting point of 1063.degree. C. and is superior to aluminum in strength under a high temperature.
However, copper is easily oxidized under a high temperature. A copper alloy also has a similar problem. Thus, a heat-resistant conductor is formed by a nickel-plated copper wire which is made of copper having a nickel-plated surface.
However, although such a nickel-plated copper wire causes no problem when the same is used at about 400.degree. C., its conductive property is reduced under a higher temperature due to diffusion and alloying of copper and nickel, When the wire is used at 600.degree. C. for 2000 hours, for example, its conductivity is reduced by about 20%. While platinum and gold have no such problem, it is inadvisable to put these materials into practice since the same are extremely high-priced.